Electronic stereo vision is frequently used for mapping three-dimensional surface structures in a variety of applications. These applications range from mapping of biological tissue surfaces during surgery to mapping of objects and terrain seen in aerial reconnaissance photographs.
Current systems for stereo-optical surface-mapping generally require a calibration phase be performed prior to surface-mapping, where the calibration phase uses the same lens configuration, including the same optical magnification and focal length, as used during the surface-mapping. Some stereo-optical surface-mapping is done with optical systems having fixed-magnification lens systems, including robotic systems for maneuver, parts placement, and inspection, and for aerial reconnaissance. Other optical systems, including some microscopes, may have rotary turrets where the lens system has a finite number of predetermined magnification levels and focal lengths, each of which can be calibrated separately. Typically, calibration entails calibrating the intrinsic and extrinsic parameters of the stereovision system by taking a stereo image pair, and matching a set of points with known 3D coordinates with their counterparts found in the two stereo images. A perspective projection matrix can then be established to reconstruct the 3D target surface following image rectification and triangulation of correspondence points. These techniques work best with constant, known, image magnification and focal length. Where optical zoom lenses are involved, prior systems have often been limited to a small subset of possible image acquisition settings where previous calibration has been performed.
One application where stereo-optical surface-mapping has been used to map surface features of biological tissues is brain surgery. When the skull is opened, the very soft brain tends to deform. The deformation may cause an apparent shift in location of structures that may have been mapped during preoperative imaging such as may have been performed with magnetic resonance imaging (MRI) and/or computed tomography (CT) scans. While these structures may be located with intraoperative MRI or CT scans, such scans are cumbersome to perform and repeated scans may be required as tissue shifts with repositioning of retractors.
Non-contact, stereo-optical surface mapping of the exposed brain has been performed to create an intraoperative surface map, such as a post-skull-opening surface map of brain tissue. This surface map has been used with a brain deformation model to determine post-opening, and post-tissue-retraction, locations of structures, including tumors, that were identified and mapped during preoperative imaging but shift in position as the brain deforms during surgery. A PHD thesis describing how a surface map is used with a brain deformation model may be found as Hai Sun, Stereopsis-Guided Brain Shift Compensation, A Thesis, Thayer School of Engineering, Dartmouth College, Hanover, N.H., May 2004, (Hai Sun) the contents of which are incorporated herein by reference.
Many existing surgical microscopes have optical zoom lenses instead of rotary turrets, these microscopes offer a continuously-variable range of magnification and focal length instead of a finite number of predetermined levels, and a surgeon may wish to change magnification and focus during surgery. It is desirable to adapt a surface-mapping system to mount to existing optical ports of such a microscope, and to ease acceptance of such a system by permitting the surgeon to change magnification, focus, and optical axis of the microscope while providing adequate surface mapping.